1. Field of the Invention
The present invention relates to a passive optical network system, an optical line terminal and an optical network unit, and particularly to a passive optical network system in which communication is performed by wavelength division multiplexing in an access network using an optical technique or an optical access system using a PON (Passive Optical network) system, an optical line terminal and an optical network unit.
2. Description of Related Art
In recent years, as the Internet becomes widespread, user's methods of using the Internet are diversified. In addition to mail and WEB access, file download by Peer to Peer (P2P) and viewing of movies on the network become general, and from now, broadcast is expected to be performed through the Internet. With this, a demand for speed-up of the network is raised, and Asymmetric Digital Subscriber line (ADSL), Broadband PON (B-PON), Gigabit Ethernet PON (GE-PON) (Ethernet is a registered trademark), or Gigabit Capable PON (G-PON) comes into wide use. Especially, in the PON system, when a local switch (OLT: Optical Line Terminal) installed in a station and a network unit (ONU: Optical Network Unit) installed in each user's house are connected to each other, one fiber is taken out from the OLT and is branched using an optical splitter to connect each user. Thus, the cost of laying the fiber is low, and high-speed communication can be performed since the optical transmission is used, and therefore, the PON system has become widespread in various countries in the world.
Optical transmission systems includes Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), Code Division Multiplexing (CDM) and the like. The B-PON, GE-PON, or G-PON is a system which uses different wavelengths in upstream and downstream and in which with respect to communication between the local switch (OLT: Optical Line Terminal) installed in the station and the network unit (ONU: Optical Network Unit) installed in each user, the communication of signals is performed by time division (TDM) in which signal communication time is assigned to each ONU.
In addition to the optical access system, a further high-speed PON system has been considered. As an approach to speed-up, a system of further increasing communication frequency in the TDM system, a system of increasing the speed by the CDM, or the like has been considered, and as another effective system, a speed-up system by the WDM has been considered.
In the WDM system, the OLT and the ONUs are connected through waves of plural different wavelengths in both an upstream signal and a downstream signal, and each of the ONUs receives and transmits a specific wavelength so that the communication is performed. The individual wavelength is assigned to each ONU from the OLT and the communication is performed, so that a communication band can be remarkably improved, and therefore, the WDM system is expected as a next generation optical access system.
Besides, there is disclosed an example of initial setting of a path on the B-PON system (see, for example, U.S. Pat. No. 6,097,736). Further, there is disclosed a system in which communication between the OLT and the ONUs is performed by the TDM in which signal communication time is assigned to each of the ONUs (see, for example, ITU-T Recommendation G.984.3).
In the WDM system, as described above, the communication band can be improved. On the other hand, one of future applications in the Internet is Internet broadcast. The feature of the broadcast is that the same information is simultaneously distributed to the respective ONUs, and viewing is performed by an IP function-supporting TV or a personal computer connected to each of the ONUs.
In the PON of the WDM system, in the case where a broadcast is delivered from the OLT to the ONUs, since the wavelengths of lights received by the respective ONUs are different from each other, it is necessary that the OLT copies and transmits the broadcast signal for the respective waves.
In the case where waves having the communication capacity of 1 Gbps are assigned to the respective ONUs one by one, for example, when the IP broadcast of 100 ch (about 10 Mbps per 1 ch) is assigned, 1 Gbps is used for the transmission of the broadcast. A band which can be used by the user for the other communication is not left. As stated above, when the IP broadcast is realized on the WDM-PON, the band is oppressed.
Further, in order to transmit the IP broadcast signal to the respective wavelengths, the OLT is required to have a function of copying the IP broadcast and multiplexing it to the respective waves, and the scale of this circuit becomes very large.
On the other hand, in the WDM-PON, each of the ONUs is requested to select a specific wavelength in order to connect with the OLT and to start the communication. When an ONU is installed, it is troublesome that an installer sets a wavelength other than those used in the OLT and the other ONUs. For convenience of the work, it is necessary that each ONU automatically negotiates with the OLT and is connected thereto. However, in the initial state, since it is difficult for the ONU to know which wavelength is to be used to perform the connection, each ONU can not perform the communication for negotiating with the OLT about which wavelength is to be used to perform the communication.